lonomer compositions may be made by blending individual polymer materials. The blending can give new property combinations. lonomer compositions obtained by the blending of an ionomer with other polymers have found applications in different areas e.g. heat seal compositions (see for example EP 94 991) and toughened polyamide combinations (see for example U.S. Pat. No. 4,078,014).
Packaging materials using blends involving ionomers are disclosed in JP 3092-340-A and JP 8205765-A. Golf ball related prior art includes U.S. Pat. No. 4,274,637; JP 1207-343-A; US 4,323,247; GB 2134799A; WO 87/02039-A; AU 8930892A=U.S. Pat. No. 4,990,574; JP 600905756A and U.S. Pat. No. 4,984,804.
U.S. Pat. No. 3,819,768 discloses a golf ball cover derived from 90 to 10 percent of an ionic copolymer of an unsaturated monocarboxylic acid and from 10 to 90 percent of an ionic copolymer of an olefin and a zinc salt of an unsaturated monocarboxylic acid. All Examples use blends of Surlyn (Registered Trade Mark) which are ethylene-methacrylic acid copolymers.
JP-1987 - 63468 discloses a mixed ion golfball blend having low degrees of neutralisation by divalent cations and high melt index precursors.
U.S. Pat. No. 4,911,451 discloses analogous blends but using ethylene-acrylic acid copolymers. The sodium ionomer is derived from a copolymer which contains 11 wt % of acrylic acid and is neutralised to about 40%. The zinc ionomer is derived from a copolymer having from 11 to 16% of acrylic acid neutralised to an extent of from 10 to 40%. A series of Escor (Registered Trade Mark) materials are used which have an appropriate melt index. The overall degree of neutralisation provided by zinc is less than 23%. No extrusion process is disclosed. EP 443 706 also discloses blends of ethylene-acrylic acid ionomers made using cations of both sodium and zinc. The total neutralisation degree is from 30 to 60%, with at least 10 wt % neutralised by divalent metal ions. In EP 443 706 the melt index of the cross-linked ionomer is from 0.5 to 5.0 g/10 min. Before neutralisation the precursor has a melt index of from 20 to 150 g/10 min. Neutralisation with individual ions is also specified at from 20 to 70% of the carboxyl groups with alkali metal ions. For the ionomer neutralised with a divalent metal ion, the neutralisation is from 25 to 70 mol % of the carboxyl groups. A broad range of blend ratios (from 80/20 to 30/70) is disclosed. Stiffness is also stated to be a significant performance parameter. The zinc/total cation ratio in the Examples does not exceed 0.3 for EAA ionomer blends. No extrusion process is disclosed.
U.S. Pat. No. 5,120,791 also discloses ionomer blends. Examples 6, 20 and 23 blends EAA or E-MA-AA ionomers which are significantly neutralised. No mention is made of the existence of or the extent of a melt index uplift. The degree of neutralisation is fairly low. The precursor melt index is fairly high.
lonomers are sensitive to moisture levels. If these are too high an increase in melt index may result which is generally indicative of a deterioration in ionomer quality which may lead to surface defects such as bubble formation in for example films.
In the use of ionomers, the melt index of the unneutralised precursor influences the processability of the neutralised ionomer. Too high a molecular weight and the processability may be restricted. For good processability, lower molecular weight precursor may have to be used even though these are otherwise less desirable.
It has now been found that a significant increase in Melt Index (expressed herein in terms of the "Melt index uplift") may result for an ionomer, which increase goes beyond that normally deemed possible with either divalent or monovalent neutralising cations, by using a combination of monovalent and divalent cations at given ranges of cation ratios and neutralisation degrees. The melt index uplift can be used to improve processability forgiven levels of cross-linking in ionomer or to work with higher molecular weight precursors or work with higher cation levels whilst maintaining processability and so provide improved properties.
The MI uplift is not attributable to water content.